Conventional controllers in air conditioning systems may be of the general type shown in: U.S. Pat. No. 4,077,567, entitled Pneumatic Temperature Reset Differential Pressure Controller; and U.S. Pat. No. 4,042,173, entitled Method and Apparatus for Controlling Volume Air Flow. Generally such controllers are of the pneumatic type and employ duct or independent supply air in the control and operation of a pneumatic actuator which moves a flow regulating damper in response to the demands of the controller. A differential pressure across a fixed orifice is conventionally employed to provide a flow variable signal representing flow in the duct section associated with the damper, the differential pressure being applied across a diaphragm for operating a push rod or other output member in a pneumatic-mechanical motion transducer. The push rod is operatively associated with a pivotally supported lever which is also influenced by a reference or bias signal provided by an adjustable reference means normally in the form of a spring engaging the lever. When temperature control or temperature reset is introduced, a pneumatic thermostat is usually employed with a pneumatic-mechanical motion transducer having a push rod or other output member acting in opposition to a bias or reset spring. The temperature or other air condition signal is combined with the aforesaid reference or bias signal to provide a composite signal acting on the lever in opposition to the push rod or other output member of the flow or differential pressure transducer. The temprature or other air condition control or reset signal may be transmitted to the reference or bias spring by a second pivotally supported lever.
The first mentioned lever of the controller provides a resultant signal at a control orifice which may comprise a flow variable signal compared against a reference or bias signal or a flow variable signal compared against a composite signal comprising the reference or bias signal and a temperature or other air condition signal. The control orifice thus establishes a pneumatic actuator operating pressure for movement and control of the damper for the maintenance of desired flow or volume conditions in the duct and for reset operation in accordance with the temperature or other air condition signal.
Controllers of the type described operate efficiently in regulating conditions in a single duct, mixing or air distribution box. They do not, however, efficiently meet requirements where two or more duct sections, mixing or distribution boxes must be coordinated in their air flow or volume control. For example, where supply and exhaust ducts or boxes require coordinated flow control, independent control by two or more controllers of the type described may result in excessive variation and departure from the desired coordinated flow conditions. Reference or bias and reset springs, lever pivot friction, transducer push rod friction, etc., result in inaccuracies unacceptable for certain applications. Further, the necessary flexibility in coordinating air flow or volume is lacking. A one-to-one flow rate might be practical with a pair of independent controllers if ideal conditions prevail in the design and manufacture of the controllers but various other flow ratios would be difficult if not impossible to achieve with the necessary degree of accuracy. Many applications require flow ratios other than one-to-one.
It is the general object of the present invention to provide an improved air conditioning control system employing a master controller of the type described in combination with a tracking controller capable of closely and accurately following the operation of the master controller and its actuator and damper and capable further of convenient adjustment of flow ratios between the dampers and duct sections, mixing and/or distribution boxes associated with the two controllers.
A further object of the invention resides in the provision of a tracking controller capable of use with a variety of types of master controllers and in other systems without master controllers but where coordinated flow in two or more duct sections is required.